This invention relates generally to amplifiers, and more particularly to an operational amplifier having an improved gain bandwidth product.
In the case of an operational amplifier utilizing single pole compensation, the open loop gain is generally larger at a low frequency and decreases to unity at the unity gain frequency (f.sub.u) at a rate of 20 db per decade. The gain bandwidth product (GBW) for such a single pole slope is a product of frequency (f.sub.1) and the gain (A.sub.1) at f.sub.1 ; i.e. GBW=A.sub.1 .multidot.f.sub.1 =f.sub.u. There may be applications, however, where for operation at f.sub.1, a gain greater than A.sub.1 is desired. Merely increasing A.sub.1 would result in substantially increasing the unity gain frequency which in turn may cause the amplifier to be unstable in a closed loop unity gain configuration.
It is well known that the gain A.sub.1 can be increased without increasing the unity gain frequency by adding a pole/zero transfer function to the forward signal path. Generally, the pole is at a lower frequency and the zero is at a higher frequency and both frequencies are less than the unity gain frequency. This combination permits a reduction in the unity gain frequency while at the same time cancelling the undesirable phase introduced by the added pole.
Copending U.S. patent application Ser. No. 880,475 filed June 30, 1986, entitled AMPLIFIER HAVING IMPROVED GAIN BANDWIDTH PRODUCT and assigned to the assignee of the present invention discloses a circuit wherein a pole/zero doublet is added into the transfer function such that the zero (Z.sub.1) appears at a frequency twice that of the added pole (P.sub.1) doubling the gain bandwidth product relative to the unity gain frequency. That is, GBW equals 2f.sub.u. It would be desirable, however, to produce a system wherein the added pole may be reduced in frequency by greater than a factor of two, for example in AC shorted current mirrors, thus bringing about a correspondingly greater improvement in gain bandwidth product.